Many medical systems are used for guiding instruments by means of position sensors and ultrasonic probes. For example, U.S. Pat. No. 6,216,029 to Paltieli describes a system with a position sensor on an ultrasonic probe, which is used in conjunction with a needle, or other insertion device, on which another position sensor is attached. The absolute location and orientation of the plane displayed by the imaging system is determined by the position sensor on the ultrasonic probe (also called ultrasonic transducer, the terms being used interchangeably). The system enables the measurement of the relative location and orientation of the needle with respect to target tissue. Once the spatial data are determined, it is possible to derive the expected path of the needle towards the target and to display it on the image in order to enable the physician to navigate the needle precisely towards the target.
In such a system, the needle position sensor, secured to a predetermined point on the needle, measures the precise location and orientation of the needle upper tip but the ultrasonic position sensor, being attached to the ultrasonic transducer at a convenient, arbitrary location thereon, does not have a well determined spatial position and orientation to the scan plane of the transducer so as to precisely relate the transducer position sensor to the transducer scan plane. Yet, since the navigation of the needle to the target uses the ultrasound image as a background for the display of the future path of the needle, it is imperative to calculate the precise location and orientation of the scan plane with respect to the position sensor on the ultrasound transducer.
U.S. Pat. No. 6,604,404 to Paltieli et al. describes systems for calibrating a first position measuring component on an imaging or scanning transducer with respect to the scanning plane. Calibrations are performed by using a calibrating device including an additional or second position measuring component, such that during the calibration process, the relative position of between these position measuring components can be calculated. Calibrations are also performed by viewing targets on the scanning plane that are at a known position with respect to the second position measuring component. Calibrations are also performed based on the scanning plane and position measuring component on a guided device, such as a needle, that typically is used in conjunction with the imaging or scanning transducer.